The present invention relates to a wafer heating device used in a semiconductor fabrication equipment, and a method of controlling the same.
FIG. 1 shows an outline of a conventional epitaxial growth system. This system is used when a thin layer of silicon is deposited on a surface of a wafer.
An susceptor 12 of annular shape is disposed inside a reaction chamber 11. A wafer 10 is supported at the edge thereof on the susceptor 12. The susceptor 12 is supported at the edge thereof by a cylindrical drum 13, and mounted on a rotary drive mechanism (not shown) via the drum 13. With rotation of the susceptor 12, the wafer 10 held thereon is rotated.
Within the drum 13, a first heater 21 is disposed in a position facing a lower surface of the wafer 10. As shown in FIG. 2, the first heater 21 is formed by arranging a heater in a spiral shape (or in a zigzag shape, or in a multiple-stripe shape), and has a disc shape as a whole. Further, a second heater 22 of annular shape is arranged so as to surround the first heater 21. The first heater 21 is used for heating the wafer 10. The second heater 22 is mainly used for heating the susceptor 12. The first heater 21 and the second heater 22 are not rotated.
A reaction gas supply nozzle 15 is provided at a ceiling portion of the reaction chamber 11. A silicon layer is deposited on the heated wafer 10 by supplying a reaction gas including silicon compound from this nozzle 15. Rotation of the wafer 10 promotes growth of the silicon layer, and improves uniformity of a thickness of the formed silicon layer.
Radiation thermometers 31 and 33 are mounted on the ceiling portion of the reaction chamber 11. A feedback control of powers of the first heater 21 and second heater 22 is performed by measuring surface temperatures of the wafer 10 and the susceptor 12 with these radiation thermometers 31 and 33.
FIG. 2 shows a plan view of the part of the first heater 21 and the second heater 22. In FIG. 2, reference numeral 21 denotes the first heater for heating the wafer, and 22 denotes the second heater mainly for heating the susceptor 12. A broken line 23 denotes an outer periphery of the wafer.
In a control method as described above, a temperature of the wafer can be accurately controlled in the vicinity of a measuring point of the temperature on the surface of the wafer 10. However, the temperature of the wafer cannot be accurately controlled in the other positions. Therefore, non-uniform temperature distribution occurs within the surface area of the wafer 10. In order to obtain an uniform thickness of a silicon layer to be formed, it is necessary to heat the wafer 10 not so as to generate a temperature difference within the surface area of the wafer 10. Large non-uniformity of the thickness of the formed silicon layer causes deterioration of quality and yield of semiconductor devices to be fabricated by using the wafer.
Recently, in order to improve chip multiprobe yield (an yield of devices per unit area of a wafer), the diameter of wafer is gradually increasing, such as 200 mm, 300 mm. In a wafer of a large diameter, it has become more difficult to heat the wafer uniformly within the surface area of the wafer.
Among factors which prevent uniform heating of the wafer, there is a phenomenon that heat flow is taken away from wafer via the susceptor supporting the edge of the wafer and that the temperature of the peripheral area of the wafer decreases. Since the susceptor has a larger thickness and a relatively larger heat capacity than those of the wafer, a large quantity of heat is taken from the wafer to the susceptor, and the temperature of the peripheral area of the wafer decreases. In order to prevent such decrease of the temperature in the peripheral area of the wafer, the second heater 22 for heating susceptor is provided.
FIG. 3 is an example of a control block diagram relating to powers of the first heater 21 and the second heater 22 in the above conventional semiconductor fabrication equipment. As shown in FIG. 3, the powers of the first heater 21 and the second heater 22 are independently controlled by separate PID-method closed loops. The power of the first heater 21 is controlled by using the temperature of the wafer 10 as a feedback signal, and the power of the second heater 22 is controlled by using the temperature of the susceptor 12 as a feedback signal. Further, when wafer 10 is not set on the susceptor, the power of the first heater 21 is fixed at a predetermined value.
A method of operating the semiconductor fabrication equipment shown in FIGS. 1-3 will be described.
The epitaxial growth system shown in FIG. 1 is a type of single wafer processing. The wafer 10 is treated one by one as follows. The wafer 10 is transferred into the reaction chamber 11 by a transfer robot (not shown). Then, a silicon layer is deposited on the surface of the wafer 10 in the reaction chamber 11. After deposition of the silicon layer has been completed, the wafer 10 is transferred from the reaction chamber 11 by the transfer robot. Thereafter, a new wafer is transferred into the reaction chamber 11, and a silicon layer is deposited again on the new wafer.
In a process of treating one wafer, a setting temperature of the wafer is program-controlled in accordance with a pattern as shown in FIG. 4, for example. In this example, the setting temperature of the wafer is set to 800.degree. C. at first, and the temperature is maintained for 1 minute. However, since no temperature of the surface of the wafer is obtained while exchanging wafers, the power of the heater 31 is fixed at a predetermined value (for example, a value at which the temperature of a wafer is expected to be stabilized at about 800.degree. C.). During this time, the wafer 10 is transferred into the reaction chamber 11 by the transfer robot (not shown) through a gate (not shown) located at the peripheral wall of the reaction chamber 11.
FIG. 5 shows an operation of the system when the wafer 10 is transferred onto the susceptor 12. As shown in FIG. 5, pins 17 ascend from under the wafer 10, and receive the wafer 10 from the transfer robot. Thereafter, the transfer robot moves back to the outside of the reaction chamber 11, and the gate is closed. The wafer 10 is heated to a temperature close to 800.degree. C. in a state of being set on the pins 17.
When the temperature of the wafer 10 has rised to nearly 800.degree. C., pins 17 descend and the wafer 10 is transferred onto the susceptor 12. Next, rotation of the susceptor 12 is started. The temperature of the wafer 10 is maintained at 800.degree. C. for 1 minute by a feedback control, as shown in FIG. 4. Then, the setting temperature value is linearly raised to 1000.degree. C. for 3 minutes. If the wafer 10 is rapidly heated, the thermal stress increases, which causes deterioration of the quality of a silicon layer to be deposited. Therefore, the wafer is gradually heated as described above.
After the setting temperature value has reached 1000.degree. C., the temperature is maintained for 4 minutes, during which a reaction gas including silicon compound is supplied onto the surface of the wafer 10. Thereby, a silicon layer is deposited on the wafer 10.
Then, supply of the reaction gas is stopped, and the setting temperature value is linearly lowered to 800.degree. C. for 1 minute. After the setting temperature has been lowered to 800.degree. C., the gate of the reaction chamber 11 is opened, pins 17 are raised, and the transfer robot is advanced, and the wafer 10 is transferred to the transfer robot.
(Problems of the conventional heater controlling method)
In the conventional heating device shown in FIGS. 1-4 and the temperature control method thereof, the temperature of the wafer is set in accordance with specification of semiconductor devices to be fabricated. At this time, it is not always easy to set the temperature of the susceptor to an optimum value. Usually, the setting temperature of the susceptor is set to be the same as the setting temperature of the wafer. However, in that case, since the heat capacity of the susceptor is greater than that of the wafer, rise of the temperature of the susceptor is behind that of the wafer. Thereby, heat flows out from the wafer to the susceptor, and the temperature of the peripheral area of the wafer decreases. Therefore, in order to prevent decrease of the temperature of the peripheral area, there are the cases where the setting temperature of the susceptor is set to a temperature which is tens degrees higher than that of the setting temperature of the wafer. If such a method is adopted, an optimum setting temperature of the susceptor needs to be determined by several trial-and-error processes.
Further, the temperature distribution in the reaction chamber changes with the lapse of the operation time of the system. Specifically, immediately after operation of the system is started, the whole system has not yet been heated, and thus a large heat flow is taken from the wafer via the susceptor. Therefore, the temperature of the peripheral area of the wafer greatly decreases. After several hours have passed since the start of the operation, the susceptor has been sufficiently heated, and the heat flow flowing out to the susceptor is reduced. Therefore, if the setting temperature of the susceptor is fixed at the temperature at which the operation was started, there are the cases where the temperature of the peripheral area of the wafer becomes higher than the temperature of the central area of the wafer (therefore, the setting temperature), by contraries.
From the above backgrounds, it is not easy to maintain the quality of the silicon layer deposited on the wafer, in a stable state for long time.
FIG. 6 shows an example of results of temperature measurement. This graph shows variations of the temperatures (indicated by the left-hand scale) of several parts of the wafer and that of susceptor, in the case where the temperature of the wafer (diameter: 300 mm) is raised from 800.degree. C. to 1100.degree. C. and maintained at 1100.degree. C. for 180 seconds. In this example, the setting temperature of the susceptor is higher than that of the wafer by 30.degree. C.
In FIG. 6, line a denotes a temperature of an area 50 mm distant from the center of the wafer, line b denotes a temperature of an area 120 mm distant from the center of the wafer, line c denotes a temperature of an area 130 mm distant from the center of the wafer, and line d denotes an area 145 mm distant from the center of the wafer (peripheral area). Further, line e denotes a temperature of the susceptor. In FIG. 6, line f denotes a temperature difference within the wafer, specifically, the difference between the maximum value and the minimum value of the temperatures a-d (indicated by the right-hand scale). The power of the first heater 21 in the central area (FIG. 1) is feedback-controlled by using the measured value of the temperature c. In the meantime, the power of the second heater 22 in the peripheral area (FIG. 1) is feedback-controlled by using the measured value of the temperature e.
As shown in FIG. 6, the temperature of the feedback-controlled area (c) is constant. However, the temperature difference between the central area (a) of the wafer and the peripheral area (d) increases with the lapse of time, and a temperature difference greater than 10.degree. C. occurs.
In order to shorten a cycle time per wafer and increase throughput, the temperature in the reaction chamber is maintained at a high temperature of a certain degree while exchanging wafers. When a wafer is transferred into the reaction chamber and set on the susceptor, if the temperature difference between the wafer and the susceptor is large, the wafer is deformed. Such deformation will be a main factor of increasing defects in crystal structure. Consequently, the deformation deteriorates the quality of a semiconductor device fabricated by using the wafer, and lowers the yield. Therefore, it is necessary to maintain the temperature of the susceptor at a proper value.
Instead of the temperature controlling method as described above, it is also possible to control the second heater 22 disposed outside (in FIG. 1) by using the temperature of the peripheral area of the wafer as the feedback signal. However, this method cannot be applied when no wafer is set on the susceptor.
Further, there is also a method of independently controlling heaters which are provided in correspondence with the central area of the wafer, the peripheral area of the wafer, and the susceptor. However, this method complicates the structure of the system and increases the manufacturing cost of the system. Further, since the peripheral area and the susceptor are adjacent to each other, interference occurs between two control loops thereof. Therefore, it is not always easy to independently control the heaters corresponding to the peripheral area and the susceptors.